Maintain stronghold (4)
I believe the answer is:
keep
I believe this is a double definition.
'maintain' is the first definition.
(I know that maintain is a more specific form of keep)
'stronghold' is the second definition.
(I know that keep is a type of stronghold)
(Other definitions for keep that I've seen before include "Stock" , "Guard" , "Retain in old castle (4)" , "Hold - fortress" , "honour" .)